Distribution network

ABSTRACT

In a distribution network system delivering services to customers, convergence of internet and asynchronous transfer mode (ATM) protocols provides common distribution of data and real time communications services in the network.

This invention relates to distribution networks and in particular to amethod and arrangement for facilitating communication in such networks.

BACKGROUND OF THE INVENTION

Data networks based on packet protocols, such as Internet protocol (IP),use addressing information in the packet header to determine thedestination of individual packets. Since each packet is treatedindependently of any other packet, the networks are connectionless.Packets pass individually through the network between router nodes whichdetermine to which router the packet should be sent next. The processingpower required to interpret packet header information and determine theroute to the next network node limits the capacity of the router.

A description of an arrangement and method for transportingconnectionless traffic is given by Grovenstein et al. in IEEE Network,the Magazine of Computer Communications, vol. 8, No. 6, Nov. 1 1994,pages 18 to 22.

In many instances, a communication between two terminal nodes in acommunications network involves transmitting many IP packets insequence. This is commonly referred to as a ‘flow’. When a flow isidentified by a router, a temporary link ‘connection’ can be set upthrough the network to carry this flow. All packets with the same headerinformation can then pass over this connection avoiding the need toprocess the headers on an individual basis. This technique is known asIP switching.

Distribution networks for providing multimedia services to subscriberterminals are currently being introduced. One proposed form ofdistribution network employs ATM (asynchronous transfer mode) as atransport mechanism, and such a network is described in our co-pendingUnited Kingdom patent applications Nos 9602808.9 9602807.1 and9602809.7. While ATM is suitable as a transport in the core network, thelow cost integration of services is at present enabled in the access andcustomer premises networks by the use of Internet technology for theintegration of voice and data. However, the Internet is not adapted toreal time communications because of its connectionless paradigm,multiple routing hops and minimum packet length. This can introduce endto end delays thus impairing the quality of service.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an improved method ofdelivering services in a distribution network.

It is a further object of the invention to provide Internet real timecommunications services.

According to the invention, there is provided a method of transportingconnectionless packet traffic together with connection oriented trafficover an asynchronous broad band (ATM) network comprising an accessnetwork incorporating a plurality of access units (11) each coupled to aswitch or core (12), the method including, providing permanent virtualchannels between said access units, multiplexing traffic from aplurality of users into a single minicell connection wherein each saiduser's traffic is carried in minicells each provided with a headeridentifying that user's channel, establishing a communication path forsaid minicell connection over a said permanent virtual channel via aadaptation negotiating procedure (ANP), and switching the multiplexedtraffic at an ATM adaptation layer associated with the ATM network.

We have found that the eveloving definition of a composite user ATMAdaptation Layer (AAL-2) in ITU SG13, may be used to provide aneffective solution to the integration of real-time communications andInternet service. We address the issues of end-to-end connectioncontrol, and integration of Internet, telephony and legacy servicesaccording to the relevant sections of CFP6.

A feature of our approach, is the ability to provide real-timecommunications services with a quality equal to that of existing PSTNsover ATM. These services may be multiplexed in the same connections inthe access and core networks with Internet protocol packets. The networkmodel is in essence both a switching network for real-timecommunications, and a router for Internet packets, handling both typesof service in a single integrated connection oriented manner.Additionally this obviates the need to perform Internet protocol routingin the ATM network.

ATM is used herein purely as a transport technology with the use ofpermanent virtual connections (PVC) in the core and access networks. Theservices are switched in the adaptation layer (AAL-2), rather than inthe ATM layer, in a so called adaptation layer switch (ALS), whichequates to the LE (local exchange) in the core network in the DAVICreference model. The ALS responds to standard adaptation negotiationprocedures (ANP) of I.363.2 to control connections, and requires noother signalling interface. A Network Adapter, which corresponds to theAccess Node in the DAVIC reference model, provides end-to-endconnectivity across the core network by using the ANP to control AAL-2connections in the core that are switched by the ALS.

The Network Adapter responds to proxy signalling and may perform aconcentration function. The Network Adapters perform adaptation ofreal-time communications and Internet to and from the AAL-2 format.Alternatively the adaptation may be performed in the STB by back-haulingthe AAL-2 format to the Service Consumer System (SCS).

Further, we have found that AAL-2 can be distributed in MPEG-2 frames,allowing it to be multiplexed directly with MPEG-2 based transport inthe access network. This extends one uniform technology from the corenetwork through to the subscriber.

In our arrangement, the AAL-2 adaptation layer of ATM provides a newparadigm for communications and packet transfer. The adaptation layerhas been optimised to cope with the demands of low-bit-ratecommunications, representing the increasing move towards greatercompression. In particular, the adaptation layer is a multiplex of usersin a single ATM connection, where each user's information is carried ina short packet, with a header identifying the user channel withancillary control information.

There are several benefits of this approach, the two main ones being

low-delay—ATM cell assembly delay is a problem that becomes more andmore pronounced with lower bit-rate, as it represents a significantchunk of network end-to-end delay budget which is planned to avoidunpleasant voice echo and good performance from interactive services. Byhaving a multiplex of short packets in the payload of a cell, so calledmini-cells, the length of those mini-cells can be tailored to thebit-rate of the service, and in general not suffer the ATM cell assemblydelay.

high link bandwidth utilisation—by multiplexing several users in aconnection oriented manner over a single ATM PVC, and taking account ofthe statistics of the service type and user call model, allows for aconcentration function that ensures a high utilisation of link bandwidthwith small or no padding overhead. The ATM connection is effectively anasynchronous dynamic trunk group.

In our arrangement, AAL-2 handles all mobile and compressed voicestandards with or without Speech Activity Detection down to bits ratesas low as 4 kb/s. It is also designed to handle long data packets bysegmentation and re-assembly to and from the short mini-cell format,such that mobile packets can be carried along with the voice.

Essentially, AAL-2 is suitable for all legacy telephony and circuitswitched wire line traffic, and all wireless services.

AAL-2 minicells may operate equally over a Switched Virtual Circuit(SVC), or in view of the connection's “trunk group” behaviour, over apermanent or semi-permanent ATM VCC. To establish user channels throughan AAL-2 based VCC, a straightforward handshake protocol calledAdaptation Negotiation Procedures (ANP) is defined in the standard. TheANP operates end-to-end over the VCC, and may be tailored to the userservice type. Call establishment and maintenance, and control over theminicell length and other service specific parameters is provided. An F7OAM flow is defined in the standard to monitor link or end-to-endperformance through multiple AAL-2 connections with intervening relaynodes. The OAM can be tailored to the service type rather than being apart of the transport layer.

To establish user channels through an AAL-2 PVC, a straightforwardhandshake protocol called Adaptation Negotiation Procedures (ANP) isdefined in the network standard. The ANP operates end-to-end over a VCC,and may be tailored to the user service type. Set-up, tear-down andmodification are available operations, with control of the mini-celllength and other service specific parameters. An F7 OAM flow is definedin the standard to monitor end-to-end performance through multiple AAL-2connections with intervening switching nodes. The OAM can be tailored tothe service type, rather than being a part of the transport layer.

In particular, we exploit the generic capability of AAL-2, which is anasynchronous connection-oriented short packet protocol, where ATMprovides virtual connectivity in the transport layer. This allowsreal-time traffic to be easily integrated on the same links with dataand to be switched. For example, the real-time traffic may be a freestatistical mixture of voice and video at any compression ratio,allowing full multicast video conferencing to be readily implemented.

A further benefit of the use of AAL-2 in the present arrangement andmethod is that, because of its multiplexed packet capability, mini-cellscan be switched between ATM connections. Thus only PVC capability in theATM core network is necessary, as all switching and concentration of aservice can be performed at the adaptation layer. Adaptation layerswitching allows a network to be scaled, and to be managed on a servicebasis. An ATM based network with adaptation layer switching means thatATM is retained as a transport technology, or as a switching technologyfor services not addressed in this manner.

The technique is applicable to distribution networks such as the networkprotocol defined by the DAVIC standard, but it is of course in no waylimited to that application.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to theaccompanying drawings in which:

FIG. 1 is a schematic diagram of a communications network according toone embodiment of the invention;

FIG. 2 illustrates the way in which the network of FIG. 1 is arranged todistribute services to customers;

FIG. 3 illustrates the AAL-2 multiplexing of minicells into ATM;

FIG. 4 shows an end-to-end connection path through the network of FIG.1;

FIG. 5 illustrates the application of AAL-2 to the network of FIG. 1;

FIG. 6 shows the network adapter and control planes;

FIG. 7 shows an AAL-2 end-to-end connection for real time andcut-through Internet service; and

FIG. 8 shows the way in which AAL-2 is multiplexed over an MPEG2transport stream.

DESCRIPTION OF PREFERRED EMBODIMENTS

The network of FIG. 1, which serves a plurality of terminal stations(not shown) comprises two portions. A number of network access units 11forms a passive access distribution network, and a core 12 performs aswitching function for carrying traffic across the network betweenaccess units. The general connection of the network to a serviceconsumer system is illustrated schematically in FIG. 2.

FIG. 3 shows the way in which the AAL-2 adaptation layer provides forcommunications and packet transfer. This adaptation layer has beenoptimised to meet the demands of low bit rate communications. Theadaptation layer is a multiplex of users in a single ATM connectionwhere each user's information is carried in a short packet with a headeridentifying the user channel with ancillary control information.

FIG. 4, which depicts an end-to-end minichannel connection in thenetwork of FIG. 1, shows two types of AAL-2 network nodes. These are arelay function or Adaptation Layer Switch (ALS) that performs nosegmentation and re-assembly (SAR), but simply switches at theadaptation layer; and a Network Adapter (NA) that is the AAL-2termination, and performs adaptation, SAR and concentration. The nodesare connected by ATM permanent virtual channels (PVC) and traffic isswitched at the adaptation layer. An ATM negotiating procedure (ANP) isused to set up communication channels. As each VCC link is managed byANP to control user channels in a robust manner, the ANP can be cascadedto form minichannels end-to-end across the network. We term such acascade a wormhole. Once a wormhole connection has been established byANP alone, AAL-2 mini-cells can be transparently routed in aconnection-oriented based on the channel identifier and associatedpriority or traffic class. A more detailed description of this routingtechnique is given in our co-pending United Kingdom patent applicationNo 9614138.7. In this technique, routing is determined from the networkperiphery by defining a succession of nodes and virtual channels (VCs)to be used to reach the destination.

An important part of this end-to-end connectivity is the Grade ofService (GoS). By providing permanent virtual channels that contain amultiplex of users, the ATM traffic contract for the PVC can bespecified as a discontinuous bit rate (DBR) service, and the bandwidthreserved and re-negotiated over an interval of time that reflectschanging aggregated demand on any given link. This essentially creates alow blocking probability for the an AAL-2 network. A network adaptor cantherefore route an individual call by ANP alone, with minimalinteraction with the ATM layer, with a high degree of success. Thelow-delay and high connectivity success rate yield a high GoS. Further,by using soft PVCs (SPVC), the network can be designed to be resilientto failure conditions.

In this network topology, ALSs are fully meshed by PVCs; these PVCs arecontained in PVPs to aggregate and smooth traffic by containing itinside a larger envelope. The PVPs link the ALSs to ATM VCcross-connects; this simplifies the connectivity as the network scales,and by having more than one cross-connect provide physically diversepath routing. The capacity of the VPs can be re-negotiated on a slowlychanging basis, according to time variations of network load. Byapplying load-balancing and soft PVCs (SPVC), the network can bedesigned to be resilient to failure conditions.

Each ALS has a complete picture of the utilisation of the VPs, such thatbetween them have a complete view of internal network congestion. ALSconnection control can provide cell based adverts, economicallythroughout the network, to the network adapters when congestion exceedscertain thresholds. The network adapter uses these congestionindications when constructing a wormhole connection to achieve a highGoS.

A network adapter plans a minicell channel connection or wormhole pathfrom the end-points of the call provided by a routing agent. End-to-endANP connectivity also allows multicast connections to be set up. Thenetwork is a generic model that allows routing decisions to be made atthe network edge. Thus, call admission control (CAC) is a feature of thenetwork adapter providing a high GoS by reserving band width accordingto traffic demand. The wormhole path takes into account a dynamicpicture of network congestion, and aims to load balance traffic to avoidcongestion conditions from occurring. The same congestion indicationscan be use to reliably predict the likely success of connecting a paththrough the network, which allows blocked traffic to be rejected atsource, thus ensuring network stability.

Wormhole connectivity also permits multi-cast connections to be set up.A network adapter can optimally choose the branch points for economy andfrom congestion indications, such that the branch point is selected tobe closest to the receiver, minimising the bandwidth requirement for themulti-cast, but also taking into account the dynamic picture of thenetwork behaviour.

The Delivery System

In the network of FIG. 1, the delivery system comprises core and accessnetworks, i.e. the switching core 12 and the network adapters 11. Theaccess network does not perform switching but may, as illustrated inFIG. 5, perform concentration and multiplexing in an access node (AN)which interfaces at A4 to the core network, and via a networktermination at A1 to the Service Consumer System (SCS). The core networkis outside the scope of the DAVIC specification, but is assumed toprovide switching and end-to-end connectivity. The connectivity iscontrolled via the network related control over an A7 interface to theaccess node, and the A5 interface to the core network.

FIG. 5 further illustrates the positioning of the two AAL-2 node typesin the DAVIC reference model. The ALS (or relay) would be located in thecore network, and therefore its exact operation is outside the scope ofDAVIC. However, the principles on which it operates, namely to provideend-to-end connectivity through the ATM based A4 interface with aguaranteed GoS, demonstrates that AAL-2 its appropriate and sufficientfor this purpose. Here we show that the network adapter has a one-to-onecorrespondence with the DAVIC access node.

DAVIC defines an access node (AN) to perform adaptation to the accessnetwork technology, concentration and proxy signalling. In the userplane, a network adapter may perform adaptation to AAL-2, or it maysimply concentrate AAL-2 traffic connections (relay), thereby extendingAAL-2 back through the access network to the SCS. In the latter event,the adaptation function would then be performed by the STB. This allowsATM PVCs to be configured between the AN and SCS, alleviating any demandfor switched virtual circuit (SVC) capability in the access network andachieving simplicity. It is in the distribution network between the A2and A3 interfaces, where AAL-2 achieves the greatest benefit from itslow-delay and high bandwidth utilisation.

In the control plane, the network adapter may accept proxy Q.2931signalling from the network related control (NRC) as connection requestsover the A7 interface, and use them to establish minicell channelconnections by ANP, both backwards through the access network (A3interface), and end-to-end across the core network (A4 interface). Sucha mechanism can achieve connectivity of STB to STB across a globalnetwork, or to the legacy narrow band network via interworking units(IWU) in the core network. The A5 interface between the NRC and corenetwork need not be used.

Referring now to FIG. 6, this illustrates the protocol stacks of thenetwork adaptor user and control planes. The connection control (CC)terminates the Q.2931 signalling and initiates the ATM negotiatingprocedure (ANP). Parameters such as NSAP addresses can be determinedfrom the Q.2931 signalling message. ANP is in the formative stage, andwill evolve to handle GoS and traffic class issues, but currently mayexploit Q.2931 to re-negotiate the bandwidth requirement as necessary.In a further embodiment, ANP may determine these connection parametersfrom the signalling and handle them directly.

ANP is in the formative stage, and will evolve to handle GoS and trafficclass issues, but a solution exists in the network configuration of theALS described earlier, and using Q.2931 to re-negotiate the bandwidthrequirement as necessary. In future, ANP may inherit these connectionparameters from the signalling.

Distribution of Communication Services

Here we describe how AAL-2 technology may be used to distributecommunications services in the reference model of the previous section.We illustrate the method with an example call walk through.

The call request may be initiated at the STB using the existing DSMCC UNsignalling stack, but which must be extended in any event to providespecification for communications service as a new S1 flow. To satisfythe call request, the DSMCC proxy can use the S3 flow to the servicerelated control (SRC) of the delivery system, which performs thenecessary end-point address resolution for routing, and which forwardsthe connection request to the network related control (NRC) using theexisting Q.2931 protocol over SAAL signalling stack. The NRC establishesthe connection by signalling to the access node (network adaptor) viathe A7 interface. The connection is established as described in theprevious section. The end-point access node using appropriate means toconnect through to the called party's STB, or and end-point IWU caninterwork with existing narrow band networks. In this manner it ispossible to provide full legacy interworking. Any existing telephonyservice can be applied to this model and still operate with legacynetworks. Voice quality is preserved, since the AAL-2 is tailored toprovide a low-delay interconnect, and is a fully effective technologyfor keeping to established network delay budgets. The call admissionpolicy of the access node and the connection-oriented behaviour of AAL-2ensures the GoS for 64 kb/s, P×64 kb/s, and even when sophisticatedcompression algorithms incorporating speech activity detection (SAD) areused. Furthermore, the AAL-2 standard automatically handles the holdoverdelay of ATM cells when fewer than the requisite number of calls to fillthe cell payload are in progress. This effect would be potentially moreprominent in the access network rather than post-concentration in thecore, but the slight waste of bandwidth disappears as more calls areadded to the connection, which can generally sustain N×bit-rateconnections with no further overhead.

This approach can also be extended to conferencing capabilities. Theprevious section described how the wormhole can branch connections inthe network, which can be controlled incrementally over the course of acall or at the outset. The multi-cast is handled in the ANP andadaptation layers, and only requires point-to-point connectivity ofQ.2931. The DSMCC definition would of course need to be extended in anyevent to support point-to-multipoint signalling.

Distribution of Internet Services

Whilst ATM may be suitable as a transport in the access and corenetwork, low-cost integration of services is enabled in the customerpremises network by using Internet technology as an umbrella for theintegration of voice and data. However, Internet does not readily lenditself to real-time communications because of its connectionlessbest-effort paradigm, multiple routing hops and rather long minimumpacket length. Some of these issues are being addressed by developmentof the RSVP bandwidth reservation protocol, but essentially reservingbandwidth is turning the Internet into a connection-orientated network,since it is applied at routing nodes.

We have found that AAL-2 is suitable for carrying Internet real-timecommunications services, and operating transparently to those services,yet providing the necessary low-delay and synchronisation. We illustratethis by the following description of an Internet based call walkthrough.

The STB can use RSVP to effectively establish a connection in theadaptation layer. DSMCC would need to be extended in any event to proxyor to direct the RSVP as an S2 flow to the Service Related Control(SRC). The SRC has access to the address resolution protocol (ARP) todetermine the Internet routing and to provide translation to and accessnode and STB end-point. The SRC can then signal using the Q.2931signalling protocol as for the communications services to establish aconnection through the network. Further RSVP during the course of theconnection may be initiated to control the bandwidth, and the SRC canelect to forward these as Q.2931 signalling messages.

The Internet routing is performed outside the domain of the ATM network.ATM and AAL-2 are transparent to the Internet, and vice-versa; thenetwork functions as a super-router. However, the benefit is that theGoS can be guaranteed for the real-time Internet services in a manneridentical to that of the general communications distribution.

Internet services can be interfaced at the network termination (NT) orthe STB, with LANs in the customer premises. There is no change totreatment of Internet traffic by LAN based equipment, although thiscould be extended in due course to ATM based technology.

The concept may be further extended to handle certain connectionlessInternet data packets as illustrated in FIG. 7. In certain transactionsthe packets, although connectionless, can represent persistentconnections such as file transfer. These packets, which represent aflow, put an inordinate load on routers, and would benefit from aconnection-orientated paradigm. If the traffice is directed to an IPswitch, than that switch can form a proxy signalling agent to theNetwork Related Control (NRC) and elect to “cut through” the connectionto the AAL-2 reference model by establishing a connection end-to-endthrough access and core, and use the long packet segmentation andre-assembly service specific convergence sub-layer of I.363.2. An IPswitch can instruct an STB to act as the proxy agent to tear-down theconnection when the flow ceases.

A further possibility is to use JAVA applets to extend the use ofbrowser software in the STB for example. If the applet graphicallyrepresents a menu of service provision locations or communicationsdestinations, then it can be pre-loaded to issue the necessarysignalling messages to act as a proxy to the access node to establishend-to-end AAL-2 connections.

Complete Service Integration

AAL-2 can make efficient use of all the DAVIC part 8 access distributiontransport layers. DAVIC has recommended a range of access technologies,and in particular hybrid fibre coax (HFC), and various radio accesssystems, which are specified to be MPEG-2 TS or ATM based. In thissection we demonstrate how AAL-2 may be multiplexed directly into MPEG-2TS type frames, such that it may be multiplexed with MPEG-2 delivery inthe access network, when ATM transport is not provided.

Since an AAL-2 VCC is intrinsically a multi-user connection, whenoperating on a point to point link with no ATM switching, (there beingno switching in the DAVIC access reference model), the ATM header VP andVC are effectively redundant. We note that to further maximise bandwidthutilisation, the ATM headers could be suppressed. However the point isthat the AAL-2 flow forms a self-delineating adaptation layer as it isalready asynchronous to the ATM cell boundary.

FIG. 8 shows the assembly process of MPEG-TS frames with AAL-2minicells. Because a minicell is limited in length by the standard tomaximum of 64 octets, the first minicell boundary can never be furtherthan this from the start of the frame. Consequently, the existingdefinition for the Start Field (STF) of AAL-2, which contains an offsetto the boundary, can be used as the first octet in the frame todelineate this boundary in a manner equivalent in mechanism androbustness to the current AAL-2 standard.

It will be understood that the above description of a preferredembodiment is given by way of example only and that variousmodifications may be made by those skilled in the art without departingfrom the spirit and scope of the invention.

What is claimed is:
 1. A method of transporting connectionless packettraffic together with connection oriented traffic over an asynchronousbroad band (ATM) network comprising a non-switching access networkincorporating a plurality of access units (11) each coupled to a switchor core (12), the method including, providing permanent virtual channelsin the core network between said access units, multiplexing traffic froma plurality of users into a single minicell connection wherein each saiduser's traffic is carried in minicells each provided with a headeridentifying that user's channel, establishing a communication path forsaid minicell connection over one of said permanent virtual channels viaan adaptation negotiating procedure (ANP), and switching the multiplexedtraffic at an ATM adaptation layer associated with the ATM network.
 2. Amethod as claimed in claim 1, wherein end-to-end connectivity isprovided across the network.
 3. A method as claimed in claim 2, whereinend-to-end connections are set up by proxy signalling.
 4. A method asclaimed in claim 3, wherein said proxy signalling is performed fromsubscriber terminals or network services whereby to provide homogeneouslow delay connectivity for real time communication services.
 5. A methodas claimed in claim 4, wherein said adaptation layer is an ATM type two(AAL2) adaptation layer.
 6. A method as claimed in claim 5, wherein saidconnectionless packet traffic comprises a flow.
 7. An asynchronousbroadband (ATM) network for transporting connectionless packet traffictogether with connection oriented traffic, said network comprising anon-switching access network incorporating a plurality of access unitseach coupled to a switch or core, the network including means forproviding in the core network permanent virtual channels between theaccess units, means for multiplexing traffic from a plurality of usersinto a single minicell connection wherein each said user's traffic iscarried in minicells each provided with a header identifying that user'schannel, means for establishing a communication path for said minicellconnection over one of said permanent virtual channel; via an adaptationnegotiating procedure (ANP), and means for switching the multiplexedtraffic at an ATM adaptation layer associated with the ATM network.